Electron microscopic studies of Group A streptococci have indicated that M-protein molecules appear as hair-like projections on the cell wall. On the assumption that these may be composed of subunits held together by non-covalent interactions, various chaotropic agents were used to attempt to extract the M-protein from the cell wall. Using a non-ionic detergent, it was found that large amounts of type 6, 12, and 14 M-proteins could be removed from isolated streptococcal cell walls. Purification of the type 6 protein was easily accomplished yielding a highly purified preparation which was homogeneous and very reactive with type-specific antisera. Similar extractions and purifications will be accomplished with other streptcoccal M types and various studies will be complished. 1) The physical and chemical structure of the M- proteins will be established by a number of physical and chemical parameters, including gel analysis, ultracentrifugation, peptide mapping, as well as amino acid analysis and sequencing. 2) M-protein labelled with 125I will be used to develop a reliable radioimmune assay to detect anti M- antibodies in human sera. Preliminary data suggest that this approach is possible. 3) It would be essential to know also if this antigen elicits the production of antibodies in rabbits which have the ability to neutralize the anti-opsonic effects of M-protein. These studies could be tested in vitro by bacteriocidal tests or in vivo by mouse protection assays. 4) Finally, by various in vitro and in vivo analyses, we wish to determine if our preparations of M-protein elicit delayed hypersensitivity reactions prevalent in acid extracted material, as well as any possible cross-reactivity with histocompatibility and cardiac antigens. This final study would be essential if this type of preparation is to be considered for future vaccine studies.